a) Field of the Invention
The present invention is directed to an optical scanning system. An optical scanning system according to the present invention is applicable to an optical scanning apparatus which has a function to correct facet error.
b) Description of the Related Art
An optical beam printer, a digital copying machine or the like usually utilizes an optical scanning apparatus which deflects luminous flux from a light source at a constant angular velocity by "an optical deflector which includes a deflecting reflection facet" such as a rotation polygon mirror, a rotation uni-facial mirror and a rotation dual-facial mirror and which thereafter focuses deflected luminous flux by the optical scanning system on a surface-to-be-scanned as a light spot.
For instance, as an optical scanning apparatus which deflects luminous flux from a light source apparatus and focuses deflected luminous flux by an imaging lens system on a surface-to-be-scanned as a light spot, various types of optical scanning apparatus are conventionally known for use in an optical beam printer. FIG. 5 shows two typical examples of such an optical scanning apparatus.
In FIG. 5(A), a light source apparatus 510 comprises a semiconductor laser and a collimating lens which collimates laser luminous flux from the semiconductor laser into parallel luminous flux, for example, to emit parallel luminous flux. The emitted parallel luminous flux impinges upon and is reflected at one of deflecting reflection facets 521 of a rotation polygon mirror 520 which serves as a deflector, whereby the parallel luminous flux is deflected at a constant angular velocity while the rotation polygon mirror 520 is rotated.
The deflected luminous flux is then focused by an imaging lens system which is made of an f.sub..theta. lens 531 and a long cylinder lens 533 on a surface-to-be-scanned 540 as a light spot. Thus, an optical scanning operation is performed at a constant speed as the luminous flux is deflected.
In general, in an imaginary situation in which an optical path from the light source apparatus to the surface-to-be-scanned is linearly developed, a direction which is parallel to a primary scanning direction is referred to as "a main scanning parallel direction" while a direction which is parallel to a sub scanning direction is referred to as "a sub scanning parallel direction."
In FIG. 5(B), the parallel luminous flux emitted from the light source apparatus 510 is converged only in the sub scanning parallel direction by the cylinder lens 511, of an imaging optical system forming a slit image, and focused as "a long slit image which extends in the main scanning parallel direction" in the vicinity of the deflecting reflection facets of the rotation polygon mirror 520. The luminous flux is reflected and deflected by the deflecting reflection facets to become a deflected luminous flux and is focused by the imaging lens system which is made of the f.sub..theta. lens 532 and the long cylinder lens 534 on the surface-to-be-scanned 540 as a light spot. The light spot scans the surface-to-be-scanned 540 in the main scanning parallel direction.
The surface-to-be-scanned indicated at 540 in FIG. 5 is an imaginary plane on which the deflected luminous flux is focused as a light spot. In general, a photosensitive medium which has a photoconductivity is placed so that its surface coincides with this surface-to-be-scanned.
Here arises a problem known in the art as "a facet error," that is, a variation in the position of an optical scanning line in the sub scanning direction. In a rotation polygon mirror, a facet error is created due to "variations" among a plurality of deflecting reflection facets of the rotation polygon mirror in terms of their angles to a rotation axis of the rotation polygon mirror which are supposed to be parallel. In a rotation uni-facial mirror and a rotation dual-facial mirror, a cause of a facet error is "unsteadiness" of a rotation axis of the rotation uni-facial mirror or the rotation dual-facial mirror.
A widely known solution to correct a facet error is to focus luminous flux from a light source in the sub scanning direction as a long slit image which extends in the primary scanning direction in the vicinity of a deflecting reflection facet while forming an optical scanning system so that the position of a deflecting reflection facet and the position of a surface-to-be-scanned are in geometric optical conjugation in the sub scanning parallel direction.
An optical scanning system used in this kind of optical scanning apparatus is "an anamorphic" imaging optical system whose power in the main scanning parallel direction is different from its power in the sub scanning parallel direction.
In order to perform excellent optical scanning with an optical scanning apparatus, it is necessary to ensure that "the diameter of a light spot does not largely change depending on an image height" and that "the moving speed of the light spot is constant."
A change in the diameter of a light spot depending on an image height is attributed to curvature of field within the optical scanning system. If the diameter of a light spot is changed in dependence on an image height, a resolving power of an image which is recorded by optical scanning varies. Although a change in the diameter of a light spot in the main scanning parallel direction can be corrected by electrically controlling a signal which is transferred with deflected light, a change in the diameter of the light spot in the sub scanning parallel direction can not be corrected in a similar manner.
As is well known, the equality in the moving speed of a light spot has a dependency on "the f.sub..theta. characteristic" of an optical scanning system. In the case where the equality in the moving speed of a light spot is poor due to insufficient correction of the f.sub..theta. characteristic, a distortion of a recorded image will be generated.
Hence, to realize excellent optical scanning, the f.sub..theta. characteristic and curvature of field in the sub scanning parallel direction of an optical scanning system must be excellently corrected. However, it is not always easy to satisfactorily correct both the f.sub..theta. characteristic and curvature of field in the sub scanning parallel direction of an anamorphic lens. For example, in an optical scanning system disclosed by Japanese Patent Unexamined Publication No. SHO 61-275814, a surface for correcting curvature of field in the sub scanning parallel direction is complicated in configuration, so that manufacturing a system of the lens is expensive.